1. Field of the Invention
This invention relates to a vacuum pump. The pump according to the invention is primarily used to create a vacuum insulation for walls and doors of refrigerators and freezers.
2. Description of the Related Art
Previously suggested vacuum insulations for this purpose--see for instance SE 90937, EP 188806, U.S. Pat. No. 4,668,555--comprise powder or cellular materials which are placed in a diffusion-tight receptacle which is evacuated and sealed before it is placed in the wall or door panel of the refrigerator or freezer. It is, however, time consuming to continue with the evacuation procedure as long as is desirable, which means that the procedure is not particularly well suited for mass production. There is also a risk that leakage occurs in the diffusion-tight layer during the lifetime of the refrigerator, which is about 15 to 20 years, which means that the contribution which the vacuum gives to the insulating capability disappears.
In order to create a vacuum for this type of equipment, conventional vacuum pumps are used. For instance DE 157471 describes a one step evacuation pump having a piston reciprocating in a cylinder, the cylinder and piston being provided with a check valve system by means of which the air is evacuated through an oil reservoir to the atmosphere, whereas the oil is partly returned to the lower part of the pump where it is used to eliminate dead space. However, this type of vacuum pump is expensive, large, comparatively power demanding and cannot be used to evacuate water vapor or to continue the evacuation procedure to a particularly great extent.